A radiommunoassay for multiplication stimulation activity (MSA) has been developed. Using this RiA, MSA has been measured in rat serum and found to be three-fold lower in hypophysectomized (growth hormone deficient) rats than in normal rats. MSA or a similar polypeptide has been shown to be produced in a fetal rat liver organ culture system by bioassay, radioreceptor assay, and radioimmunoassay. MSA has been shown to inhibit prostaglandin stimulated increases in cyclic AMP levels and adequate ayclase activity in chick embryo fibroblasts. A liver membrane preparation from the pygmy mouse exhibited normal binding capacity and binding affinity for MSA.